Medical institutions are equipped with a radiographic apparatus 51 for obtaining a tomographic image of a subject M. Such radiographic apparatus 51 includes a configuration in which a radiation source 53 that emits radiation and an FPD 54 that detects radiation move synchronously to continuously take a series of fluoroscopic images, and then the series of fluoroscopic images are superimposed to obtain the tomographic image (see FIG. 16.) In such radiographic apparatus 51, during taking a series of fluoroscopic images, the radiation source 53 and the FPD 54 move along a body axis direction of the subject as to approach to each other, thereby having the same position in the body axis direction. Thereafter, the radiation source 53 and the FPD 54 move along the body axis direction as to be spaced away from each other. Such radiographic apparatus is described, for example, in Cited Document 1. See Patent Literature 1.
The radiation source 53 has a collimator 53a attached thereto. Control of the collimator 53a can achieve limitation of a range where the radiation source 53 emits radiation. When the collimator 53a is controlled such that radiation is emitted to only a site of interest of the subject, unnecessary exposure to the subject can be suppressed. See FIG. 17.
Description will be given of the foregoing operation of taking the tomographic image by the radiographic apparatus 51. Firstly, the radiation source 53 intermittently emits radiation while moving. Specifically, the radiation source 53 moves along the body axis direction of the subject for every completion of irradiation, and again emits radiation. In this way, 74 fluoroscopic images are obtained, and then superimposed. The finished image is a tomographic image having a sectional image appearing therein when the subject is cut along a sectional plane.    [Patent Literature 1] Japanese Patent Publication No. 2002-263093
The conventional configuration as above, however, has the following problem. That is, in the radiographic apparatus 51 with the conventional configuration, a shadow of the collimator 53a appearing in the fluoroscopic image may affect acquisition of a tomographic image. Moreover, as the collimator 53a is controlled such that a radiation beam B emitted toward an FPD 54 is narrower, a range of incident radiation in the FPD 54 becomes smaller, accordingly. As a result, no radiation enters in the periphery of the FPD 54, as shown in FIG. 17.
The acquired 74 fluoroscopic images are images relating to the entire FPD 54. Consequently, the periphery of the FPD 54 where no radiation is detected is to be displayed in the periphery of the fluoroscopic image with the narrower radiation beam B. That is because the fluoroscopic images contain the entire FPD 54. Then a frame-shape dark area appears in the periphery of the fluoroscopic image acquired with the narrower radiation beam B. This dark area is a shadow of the collimator 53a falling on the fluoroscopic image.
When a tomographic image is acquired through superimposing the fluoroscopic images with the frame-shape dark areas remaining therein, the shadows of the collimator 53a are superimposed on the sectional image. The radiographic apparatus 51 takes a plurality of fluoroscopic images in various radiography directions, and generates a tomographic image based on profile views of the subject falling on the fluoroscopic images in various shapes. The shadow of the collimator 53a appears independently of the subject. Consequently, when the tomographic image is acquired through superimposing the fluoroscopic images, an image of the subject is to be reconstructed while the shadows independent of the subject are superimposed.
Then, the sectional image is blurred due to influences by the shadows of the collimator 53a. The image is extremely blurred in a boundary between the shadow of the collimator 53a and the image of the subject. More particularly, as shown in FIG. 18, the image is blurred notably at both ends of the tomographic image D in a direction where the FPD 54 and the radiation source 53 move (i.e., a vertical direction of the tomographic image D.)
This invention has been made regarding the state of the art noted above, and its object is to provide a radiation tomography apparatus for acquiring a tomographic image from a plurality of fluoroscopic images. The radiation tomography apparatus can acquire the tomographic image having superior visibility without being influenced by shadows of a collimator falling on the fluoroscopic images when radiation is applied only to a portion of an FPD through control of the collimator.